This invention relates to means for supplying matched source and sink currents to a load.
In many applications, it is necessary to drive a load by supplying a current to the load and drawing a like current from the load. A prior art circuit for performing this function is shown in FIG. 1A. The circuit includes a current sourcing circuit 12 connected between VDD and one end of a load L1 and a current sinking circuit 14 connected between the other end of load L1 and ground.
The current sourcing circuit 12 includes a current sensing resistor R1 connected in series with the conduction paths of insulated-gate field effect transistors (IGFETs) PA and NCA between VDD and an output terminal 13, to supply a load current into terminal 13 (also denoted as BPLUS). A fixed bias voltage (PBIAS) is applied to the gate of PA whereby PA functions as a constant current source. A control signal (TRI-STATE CONTROL) is applied to the gate of NCA to selectively turn it on or off. The current sinking circuit 14 includes a current sensing resistor R2 connected in series with the conduction paths of IGFETs NB and NCB between an output terminal 15 (also denoted as BMINUS) and ground. A fixed bias voltage (NBIAS) is applied to the gate of NB whereby NB functions as a constant current generator (sink), and the TRI-STATE CONTROL signal is applied to the gate of NCB to selectively turn it on or off.
The bias voltages applied to the gates of PA and NB ensure that the same amplitude current flows into the load at the BPLUS terminal and out of the load at the BMINUS terminal. A tri-state control signal applied to the gates of transistors NCA and NCB turns them both on or off at the same time. The circuit of FIG. 1 can supply (i.e. source) a current into the load which matches the current drawn (i.e. sunk) from the load.
However, the circuit of FIG. 1 does not have a high degree of power supply isolation. Power supply isolation as defined and used herein requires that there be no conduction (other than leakage) via the current sourcing and sinking circuits to VDD and/or ground when the current sourcing and sinking circuits are turned-off (i.e. disabled). Particularly, there should be no conduction via the turned-off current sourcing and sinking circuits when positive or negative signals are present at the output terminals 13 and 15 (due, for example, to inductive kicks from the load).
The lack of power supply isolation is best explained by reference to FIG. 1B which is a drawing of FIG. 1A showing parasitic and junction diodes associated with the switching and current setting transistors. For purpose of illustration, FIG. 1B shows the source-to-substrate and drain-to-substrate diodes associated with each one of the IGFETs. Consider the case where NB and NCB and PA and NCA are turned-off and a negative potential is produced at output terminals 13 and/or 15. When a signal at output terminals 13 and/or 15 goes negative with respect to ground by more than the forward voltage drop of a diode (i.e., 0.5 to 0.7 volt), diode D1NCB, which represents the subtstrate-to-drain junction of NCB, becomes forward biased and provides a conduction path between ground and the output terminals 13 and 15. Similarly, diode D2NCA which represents the substrate-to-source junction of NCA, becomes forward biased and provides a conduction path between ground and output terminals 13 and 15.
Note that if NCA were replaced by an IGFET of P conductivity type, a parasitic diode would be included as part of the IGFET which would still provide a conduction path for positive going signals at the output terminals. Therefore, the problem with a lack of power supply isolation would still exist.
In theory, diodes could be connected in series between the current source and current sink circuits and load terminals 13 and 15, respectively. However, these series diodes would cause a voltage drop which is generally very undesirable because it limits the output voltage range. In addition, the series diodes could lead to the formation of parasitic bipolar transistors which would introduce undesirable conduction paths.
It is desirable to have a current sourcing or sinking circuit with a high degree of power supply isolation. That is, a current sourcing or sinking circuit in which there is no conduction between the output terminals of the circuit and the power supply lines when the current sourcing and sinking circuits are turned off.